As President Donald Trump threatens to blow up the Iran nuclear deal, Obama administration alumni are mobilizing to defend it.

One-time aides to Barack Obama are holding meetings, contacting lawmakers and working the media in an urgent bid to prevent the dismantling of one of the former president’s signature foreign policy achievements.

And yes, Obama is well aware of their efforts.

It’s a loosely coordinated fight they successfully waged from the White House against congressional Republicans who repeatedly tried to scupper the deal. It’s also a battle they had hoped to avoid:After slamming the deal as a candidate, Trump hasalready passedup multiple chances to upend it.

But now the former Obama hands — many of whom who spent several grinding all-nighters sealing the agreement — fear Trump is prepared to sabotage it this fall.

The nuclear deal is “something concrete that is a target, something that a lot of us feel was a major achievement and strongly disagree with the critics about,” said Rob Malley, a former top adviser to Obama on Middle East issues. “And we care because of the consequences that could flow from unraveling the deal.”

Those consequences, the Obama aides stress, include a nuclear arms race in the Middle East, severe damage to America’s diplomatic standing — or worse.

“Hard to see how abandoning [the deal] doesn’t lead to war,” Obama’s former United Nations ambassador, Samantha Power, tweeted Monday.

In response, some conservatives are complaining about the return of what they called an “echo chamber” of pro-nuclear deal talking points and media coverage engineered by the Obama administration in 2015.

The 2015 nuclear deal was negotiated by the United States, Iran, China, Russia, Germany, France and Britain. It is a political arrangement, not a treaty, and it gives the Islamic Republicrelief from many sanctions in exchange for severe curbs on the country’s nuclear program. By mid-2015, U.S. intelligence officials had concluded Iran might be capable of building a nuclear bomb in two to three months.

Trump has called the agreement the “worst deal ever,” and signaled that he’s looking for a way out of it. On Thursday, the president, facing a key deadline, chose to go ahead and keep waiving nuclear-related sanctions on Iran, meaning the deal stays intact for now. That decision wasn’t a surprise given the strong advice of many Trump aides and even critics of the deal to avoid a sudden end to the agreement.

But Trump, frustrated by Iran’s non-nuclear aggression in the Middle East, has suggested that he may throw the issue to Congress by refusing to certify, by a mid-October deadline, that Iran is upholding its end of the agreement. Such “de-certification” would start a 60-day clock for Congress to decide whether to re-impose U.S. sanctions on Iran.

A vote by the Republican-majority Congress to re-impose sanctions would effectively kill the nuclear deal, and could prompt Iran to resume its nuclear program.

On Wednesday, several former Obama aides briefed reporters to warn that revisiting the nuclear deal would be impractical and dangerous. The participants included Wendy Sherman, a former State Department official who led the U.S. negotiating team pursuing the deal; Colin Kahl, who served as a national security adviser to former Vice President Joe Biden; and Jon Finer, a chief of staff to former Secretary of State John Kerry.

“It is very hard, if not impossible, to reopen this in any way without the entire deal unraveling,” Sherman warned.

Iran, for its part, has shown no interest in coming back to the table.

The nuclear deal “is not [re] negotiable. A ‘better’ deal is pure fantasy,” Iranian Foreign Minister Javad Zarif tweeted Friday. “About time for U.S. to stop spinning and begin complying, just like Iran.”

Sherman confirmed that Obama is aware of his former aides’ efforts. The former president is “engaged, staying updated” on the debate, added a current Obama aide, who declined to elaborate.

Obama is unlikely to take a more high-profile role anytime soon. He has commented sparingly on Trump’s other assaults on his legacy, including this summer’s Obamacare battle. And Trump, given his antipathy toward Obama, might feel even more inclined to scuttle the deal if his predecessor speaks up.

Also working behind the scenes is Kerry, who spent hundreds of hours personally negotiating thedeal with his Iranian counterpart over 20 months, and whose legacy is even more closely entwined with the agreement than Obama’s.

A former U.S. official who has recently spoken with Kerry said the ex-diplomat and senator remains in touch with his old colleagues in Congress, some of whom have reached out to him amid growing signs that Trump could challenge the deal. “A number have sought his thinking,” the official said.

He added that Kerry is in “frequent touch” with European officials who played a crucial role in the multinational negotiation.

“He’s said to them that they can’t let their feelings about Trump get in the way of expressing their commitment to the agreement, and he’s conveyed to them how essential their advocacy is in helping the reasonable people win the argument,” the former U.S. official said.

The Wednesday call with reporters was organized by Diplomacy Works, a group launched in May to defend the nuclear agreement. Its advisory council includes Kerry and several other top Obama-era alumni.

Other key Obama figures behind the deal are also speaking out, including former deputy national security adviser Ben Rhodes. “Trump pulling out of the Iran Deal would create a second nuclear crisis while alienating the same countries we need to address the [North Korea] crisis,” Rhodes tweeted on Thursday.

The deal’s critics include hawkish organizations, such as the Foundation for Defense of Democracies, with strong links inside the Trump administration.

Israeli Prime Minister Benjamin Netanyahu is also continuing his years-long campaign against the deal. This week, Netanyahu urged Trump to “either fix it — or cancel” the deal. The conversation will continue when Trump and the Israeli leader sit down in New York next week.

Netanyahu and other critics often focus on issues beyond the scope of the 2015 deal. Their frustrations include Iranian military activity throughout the Middle East, where Tehran has ties to Syrian dictator Bashar Assad and militias in Iraq, Lebanon, Yemen and beyond. Iran’s ballistic missile tests add to concerns about its long-term ambitions, critics say.

Trump officials and allies have made similar arguments, charging that Iran’s aggressive behavior beyond its borders violates “the spirit” of the agreement.

The fact that some of the nuclear deal’s provisions start to expire in the next decade has some critics fearing that Iran will develop a nuclear weapon soon after. The deal’s supporters respond that unraveling the deal now will definitely put Iran back on the nuclear track, and that other countries in the region, including Arab states fearful of Iran’s rise, may also try to secure such weapons.

Even many strong critics of the deal urge Trump not to quit it outright. They acknowledge the likely international backlash and note that Iran has already reaped many of the deal’s financial benefits, including access to tens of billions of dollars in frozen assets.

Many Iran hawks believe, however, that pressuring U.S. allies and using the decertification process in Congress can build momentum toward reopening or supplementing the agreement to address their concerns about its weaknesses.

Boosting those hopes are recent reports that some European leaders, such as French President Emmanuel Macron, might be open to supplemental agreements. Secretary of State Rex Tillerson raised the nuclear deal’s future with European counterparts during a visit to Britain on Wednesday and Thursday. Trump will meet in New York with Macron, White House officials said on Friday.

But Obama alumni dismiss the idea of supplemental deals as wishful thinking.

If the U.S. wants to negotiate such an add-on agreement, Kahl said, it has to offer the Iranians something new.

“You would have to bring other things to the table, not just pressure,” said Kahl, who has engaged in some notable Twitter fights with the deal’s critics.

Working in tandem with Obama alumni are activists and experts who rallied around the Iran deal in 2014 and 2015.

On Wednesday, more than 80 nuclear nonproliferation experts released a statement urging Trump not to abandon theagreement. The experts note that international inspectors have repeatedly verified that Iran is complying with the terms of the deal.

“Abandoning the deal without clear evidence of an unresolved material breach by Iran … runs the risk that Tehran would resume some of its nuclear activities,” the specialists warn.

Zarif, along with Iranian President Hassan Rouhani, are considered moderates in Iran, and they helped shepherd the 2015 deal through. Obama–era alumni warn that unraveling the agreement will only empower hard-liners in Iran, and perhaps even force leaders like Rouhani and Zarif to take more hard-line positions themselves.

France’s ambassador to the United States, Gerard Araud, has also used Twitter to dismiss any talk of reopening the deal.

“France doesn’t support the reopening of the deal. It should be rigorously implemented as it is now. How many times should we repeat it?” he wrote earlier this week, even as critics of the deal taunted him by pointing to the Macron’s reported willingness to consider supplemental deals.

Trump would also have to persuade Russia and China to get on board with a rethink of the agreement. The Chinese in particular fear that quitting or revisiting the Iran deal could lead North Korea to conclude it cannot trust America on any potential nuclear talks.

Some foreign diplomats are telling their U.S. counterparts that Trump should be careful in how far he pushes them.

“The Europeans are very clear they won’t go with us in pulling out of the Iran deal and will push back if we start going after European companies doing approved business in Iran,” a State Department official familiar with the issue told POLITICO.